The present invention relates to a method of manufacturing an electrical connection to a sheet metal part, in particular to a sheet metal part having a non-conductive or poorly conductive protective layer, using a hollow fastener element which is attached by means of a riveting process or pierce and riveting process to the sheet metal part, wherein a terminal lug is attached by means of a screw to the hollow fastener element, i.e. to the sheet metal part. The invention furthermore relates to a component assembly comprising a sheet metal part, in particular a sheet metal part having a non-conductive or poorly conductive protective layer, and a hollow fastener element riveted to the sheet metal part and designed to receive a screw which secures a terminal lug.
The requirements for electrical connections to sheet metal parts, in particular but not exclusively in the sense of earth connections to parts of motorcar coachwork prove to be increasingly more difficult.
On the one hand, high quality electrical connections are necessary, so that the ever more complicated electronic devices of motor vehicles function correctly, in particular safety-critical devices, such as for example in airbag trigger systems. On the other hand, there is an increasing trend to processing sheet metal parts which are provided with a protective layer which has only a poor electrical conductivity or indeed no electrical conductivity at all.
In the past a bolt element for the attachment of earth connections has proved itself, as is protected in the European patent EP-B-540 030.
Nowadays, an electrical connection is, however, also required which can be realised by means of a nut element. Whereas one could, in the past, rely on the fact that the attachment of a fastener element having correspondingly shaped features to a sheet metal part would result in these shaped features injuring any protective coating which was present, so that the desired electrical connection arose, it is nowadays increasingly more difficult to ensure this with sheet metal parts which are pre-painted. The corresponding protective coatings are namely so conceived that they can also participate in the most diverse shaping methods without being injured. Such coatings are not even damaged in deep-drawing processes. The danger thus exists when attaching fastener elements by riveting processes, or by pierce and riveting processes, that a non-adequate electrical connection arises between the fastener element and the sheet metal part. One therefore has to fear transition resistances arising which prevent the desired electrical function being ensured.
Moreover, sheet laminates, for example xe2x80x9cBondalxe2x80x9d registered mark of the Hoesch company) are being used nowadays, consisting of two metal layers of steel and/or aluminium with an intermediate layer of plastic. Such sheet metal laminates have very good damping characteristics are, however, only weldable under very difficult conditions, so that the production of earth connections is only possible via mechanically attached fastener elements, with an earth connection to both sheet metal components being necessary despite the insulating plastic layer which is present.
The object of the present invention is to provide assistance here and to ensure that when using hollow fastener elements, such as fastener nuts, a quality high-value electrical connection arises between a terminal lug or connection terminal and the sheet metal part, even in series manufacture.
This object is satisfied methodwise by a method of the initially named kind in that the screw which is screwed into the hollow fastener element penetrates the sheet metal part and forms or reforms a thread in the sheet metal part.
A corresponding component assembly in accordance with the invention is characterised in that the sheet metal part has a hole which is aligned with the passage of the hollow fastener element which receives the screw and the hole has a diameter or a minimum transverse dimension which corresponds at least substantially to the core diameter of the thread of the screw or is fractionally smaller than the latter.
In general, the sheet metal part is pre-holed prior to introduction of the screw for which several possibilities exist. On the one hand, the pre-holing can take place by means of a preceding hole punch during the attachment of the fastener element. In this connection, for example a modified embodiment of the method of the European patent application EP-A-0 759 510 can be used, as will be later explained.
It is sufficient to note here that in the attachment of the hollow fastener element the hole punch, which is guided concentrically to the central passage of the hollow fastener element, produces a corresponding hole in the sheet metal part before the die button supporting the sheet metal part takes care of the shaping of the sheet metal part into a hollow cavity of the fastener element.
If the hollow fastener elements are introduced into the sheet metal part in a progressive die tool set, then the pre-holing of the sheet metal part can take place in one station of the progressive die tool set, while the attachment of the hollow fastener element can take place in alignement with the so produced hole in a subsequent station of the progressive die tool set.
The sheet metal part can, however, also be post-holed, for example in a variant of the method which is described in EP-B-0 533 822 or EP-A-0 669 473. In a method of this kind, the connection of the fastener element to the sheet metal part is first brought about without piercing the sheet metal part. The follow-up tool punch is subsequently guided through the central passage of the fastener element in order to make the corresponding hole in the sheet metal part.
When using a progressive die tool set for the attachment of the hollow fastener element to sheet metal parts the attachment of the fastener elements can take place in one station of the progressive die tool set, whereas the post-holing is carried out in a subsequent station of the progressive die tool set.
In a preferred embodiment a circular hole is formed in the sheet metal part by the hole punch, and indeed with a diameter which corresponds at least substantially to the core diameter of the screw which is fractionally smaller than the latter. The sense of this design of the hole is that the screw, which can be designed as a thread-forming or thread-cutting screw, cuts or forms a thread in the sheet metal part, and optionally also in the hollow fastener element, with the thread in the sheet metal part being so formed that an intimate form-fitted connection is achieved, i.e. the sheet metal part is moulded into the thread of the screw.
A circular hole is, however, not absolutely essential. A hole punch could be used which has a polygonal or star-like or splined cross-section. A high quality electrical connection can be ensured via the screw to the terminal lug, i.e. to the corresponding connection terminal through the corners or teeth of the edge of the aperture which arises in this way. Any outward bending of the teeth or tips which arise in this way can also lead, during the attachment of the terminal lug to the sheet metal part, to an injury to the surface of the terminal lug, whereby any oxide layers are broken open and a high quality electrical connection is achieved.
Particularly preferred embodiments of the method and component assembly of the invention are to be found in the subordinate claims.
The invention will explained in the following in more detail with reference to the embodiments while referring to the drawings. The FIGS. 1 to 8 correspond essentially to those of EP-A-0 759 510 A1, however, these Figures have been modified in order to show the modified form of the hole punch, the modified form of the aperture produced by means of the hole punch and the effect on the protective coating which is present on the sheet metal part. The drawings show: